1. Field of the Invention
This invention is directed to hand held powered shears. More particularly, the invention relates to shears in which a pressurized working fluid is used to assist the operator by driving the blades with additional force as the operator controls the opening and closing of the blades.
2. Description of Related Art
There are many commercial processes requiring the use of a hand held shears to make repeated cuts. One such process is the dismemberment of poultry into its individual parts, and the present invention is particularly suited for this application, although it may also be used in many other types of cutting operations. To reduce operator fatigue, and increase the force that can be applied during cutting, the shears used in such processes are generally powered with a pressurized working fluid, such as compressed air.
One type of powered shears uses a trigger mechanism which, when depressed, causes the blades to close in a sudden single stroke. The trigger initiates the powered closing stroke, and once activated, the blades continue through the cutting cycle until fully closed. This type of shears is referred to herein as a "power operated shears". Although power operated shears are widely used, many operators prefer to have a shears where the opening and closing of the blades is totally and continuously under the operator's control.
To meet this demand, another type of shears has been developed, referred to herein as a "power assisted shears". Power assisted shears allow the operator to squeeze the blades shut or to open them, in a manner similar to the operation of a manually operated scissors, except that the blades are power driven to follow the motion of the operator's hand. This gives the operator much greater control by allowing the rate of closure and the extent of closure to be continuously adjusted during each cut.
Power assisted shears require a servo mechanism which follows the motion of the operator's hand, and which drives the blades to follow that motion. One difficulty with current designs, however, is in the placement and design of this servo mechanism. For example, U.S. Pat. No. 5,375,330 issued to Hermann on Dec. 27, 1994, discloses a power assisted shears with a servo mechanism that is externally mounted. This type of external servo mechanism has several exposed linkages which make it awkward to maneuver and difficult to clean. A smooth uncomplicated exterior on the tool is important in maintaining cleanliness and avoiding contamination during poultry or other food processing. Further, the external position of the servo requires numerous, relatively long, connecting parts and linkages which are difficult and expensive to manufacture.
Another problem with the design of some power assisted shears is that the operator must exert some force to open the blades before the power assistance of the servo mechanism is applied. The muscles used to open the hand tire much more rapidly than the muscles used to close the hand. Tools which require the operator to exert even a relatively low level of force to open the blades will fatigue the operator rapidly.
A final difficulty with earlier designs of power assisted shears has been the relatively difficult to disassemble connection between the blades, the trigger, the handle and the servo mechanism. This makes the blades difficult to remove for sharpening or replacement, and greatly decreases the likelihood of proper and thorough cleaning in the area around the trigger and blades where contamination is likely to occur.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a power assisted shears in which the servo mechanism is substantially internal, protecting it from contamination and making the tool easy to clean.
Yet another object of the present invention is to provide a power assisted shears which opens automatically when the trigger is released.
It is another object of the present invention to provide a power assisted shears wherein the blades and trigger assembly can be quickly and easily removed for sharpening or replacement and so that the tool and its components can be rapidly and thoroughly cleaned.
A further object of the invention is to provide a power assisted shears wherein the arrangement of the trigger and the blades relative to each other and to the power driving mechanism is such as to minimize tool complexity and simplify construction of the tool.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.